Total Quality Management - CII Institute of Logistics

Total Quality Management
How can we understand this curve in a practical way?
If we measure each piece that comes from a machine or operation and make a
tally of the measurements, we will eventually have a curve similar to the one
in Figure 4.3
If we don't measure, each piece, but merely reach into a tote pan, grab a
handful of pieces and measure them, the chances are that 68 out of 100
measures (34 % + 34%) will fall within the two middle sections of the graph
in Figure 4.3.
The chance are that 28 out of 100 pieces (14% + 14%) will fall into the next
two sections, one on each sides of the middle sections.
Finally, the chances are that 4 out of 100 of the pieces (2% + 2%) will fall into
the two outside sections.
This may sound a little complicated, but don't worry about it. Just remember that
measures do tend to cluster around the middle, as shown by the bell shape of the curve.
This particular curve is known as the normal distribution curve.
All of this brings us to the fifth basic principle.
5. It's possible to determine the shape of the distribution curve for parts
produced by any process
By making a tally or frequency distribution of the pieces produced by a process, we can
compare it to the blueprint specification for that dimension. In this way we can learn
what the process is doing, as compared to what we want it to do. If we don't like the
comparison, we may have to change the process or the blueprint.
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